Classifications

H04L7/02—Speed or phase control by the received code signals, the signals containing no special synchronisation information

H04L7/033—Speed or phase control by the received code signals, the signals containing no special synchronisation information using the transitions of the received signal to control the phase of the synchronising-signal-generating means, e.g. using a phase-locked loop

H04L7/0337—Selecting between two or more discretely delayed clocks or selecting between two or more discretely delayed received code signals

H04L7/0338—Selecting between two or more discretely delayed clocks or selecting between two or more discretely delayed received code signals the correction of the phase error being performed by a feed forward loop

H04L25/49—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems

H04L25/4906—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems using binary codes

H04L25/4908—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems using binary codes using mBnB codes

Abstract

A method for recoding an input sequence of words, including assigning a respective bit-grade to at least one of the bits in a first word in the input sequence, deriving candidate words from the first word in response to the respective bit-grade, and inserting one of the candidate words into each of a plurality of candidate sequences, so that each of the candidate sequences contains one of the candidate words. The method further includes adding subsequent words to the candidate sequences, the subsequent words consisting of a further candidate word derived from a further word in the input sequence, computing respective sequence parameters for the candidate sequences, based on a relation between the candidate words and the subsequent words in the candidate sequences, selecting one of the candidate sequences in response to the sequence parameters, and outputting one of the candidate words contained in the selected candidate sequence.

The present invention relates generally to data communication, and specifically to converting between serial and parallel data.

BACKGROUND OF THE INVENTION

Conversion of parallel data to serial data, termed serialization, and the converse operation, deserialization, are required for many data communication processes. The parallel data is generated on a bus, and is converted to serial data for transmission on one channel. As busses increase in width, typical busses having 64 lines or even more, the speed at which data which has been serialized needs to be transmitted must of necessity increase, to avoid data build-up at the serializer interface. Serial data rates of Gigabits/s are typically required to avoid the build-up. Multichannel SERDES (serializer-deserializer) devices comprise multiple serializers each having a serializer interface. Each interface generates a channel of serial data which is then transmitted to a receiver.

Recovery of such high speed multichannel serialized data presents considerable problems at the receiver. In systems known in the art a clock is recovered for each channel of the received data, and each clock is used to sample the received data. Typically, each recovered clock is locked to its own phase locked loop (PLL) oscillator. Furthermore, multiple sampling PLL clocks require respective elastic buffers for storing the sampled data, and there is typically an extra PLL clock for synchronizing all the sampling clocks to a common local clock.

However, each PLL may suffer from its own jitter, since it is locked to incoming data; in addition, problems are caused by the multiplicity of PLL clocks. The PLL is a highly sensitive circuit, so that in layout of a device having PLLs, each PLL is, for example, isolated as much as possible and has its own ground and supply lines. Devices requiring multiple PLL oscillators thus require more area and more pins, and typically give lower yields because one PLL failure causes device failure.

Data which is initially in an 8-bit (8b) form is typically encoded at the transmitter into an alternative form so that errors in the received data may be detected. An IEEE standard 802.3z, published by the Institute of Electronic and Electrical Engineers, New York, N.Y., describes an 8b/10b coding scheme, originally developed by IBM Corporation. Using the scheme, a transmitter maintains a table having a one-to-two correspondence, so that each 8b word may be transmitted as one of two 10b words. Each 10b word in the table has between 4 and 6 ones (and correspondingly 6 and 4 zeroes). A partial list of 8b and corresponding 10b words, according to the scheme, is shown in Table I below.

TABLE I

Decimal

First mapping B1

Second mapping

Disparity

8-bit word

value

(RD−)

B2 (RD+)

(RD)

00000000

0

100111 0100

011000 1011

same

00000001

1

011101 0100

100010 1011

same

00000010

2

101101 0100

010010 1011

same

00000011

3

110001 1011

110001 0100

switch

00000100

4

110101 0100

001010 1011

same

00000101

5

101001 1011

101001 0100

switch

00000110

6

011001 1011

011001 0100

switch

00000111

7

111000 1011

000111 0100

switch

00001000

8

111001 0100

000110 1011

same

00001001

9

100101 1011

100101 0100

switch

. . .

. . .

. . .

. . .

. . .

10111100

188

001110 1010

001110 1010

same

10111101

189

101110 1010

010001 1010

switch

. . .

. . .

. . .

. . .

. . .

11000100

196

110101 0110

001010 0110

switch

. . .

. . .

. . .

. . .

. . .

11100100

228

110101 0001

001010 1110

same

. . .

. . .

. . .

. . .

. . .

11111111

255

101011 0001

010100 1110

same

A complete listing of Table I comprises 256 rows. As shown in Table I, each 8b word is mapped to one of two 10b words. The first mapping B1 comprises words having 5 or 6 ones. The second mapping B2 comprises words having 4 or 5 ones. In transmitting a string of 8b words, a transmitter calculates a total running disparity (RD) of the string—the difference between the total number of ones and the total number of zeroes transmitted. After each 10b word has been transmitted, the transmitter evaluates if RD is positive, negative, or zero. For RD+ the following 10b word is transmitted from the first mapping B1, and for RD− the following 10b word is transmitted from the second mapping B2. If RD is zero, the fourth column, stating whether the same mapping is used or if the mapping switches, is used. The transmitter is thus able to maintain the disparity of the transmitted string within the bounds of +1 and −1.

A receiver of the encoded data is able to use the disparity properties to detect if there are errors in the received data. Typically, the receiver calculates and updates a disparity status of the received string, and if this results in a value outside the bounds, the receiver knows that there is an error in the received data. Similarly, in receiving any two sequential 10b words, if the instruction in column four is violated, there is an error in the received data. However, in most cases the receiver is not able to know in exactly which received word the error occurred. Even if it does know the exact word, the receiver is not able to correct the error.

Performance of both data transmitters and data receivers is an important factor in their operation. One of the measurements of performance is signal quality, both transmitted signal quality and received signal quality. A method for measuring signal quality, known in the art, is by generating an “eye” pattern. The eye pattern may be generated in specialized equipment by repeatedly sampling the signal level and plotting the level on a vertical axis, while triggering a horizontal axis to a signal clock. A “perfect” signal would give a rectangle, and the quality of the actual signal is proportional to the “openness” of the eye pattern generated—the more open the center of the eye, the higher the signal quality.

The specialized equipment for generating eye patterns may be available in a facility where the transmitter and/or receiver are produced, so that adjustments to the transmitter and/or receiver may be made at the facility to improve signal quality. However, such signal quality measurements and adjustments to improve the quality may not be able to be made in an “on-site” situation, because of the lack of specialized equipment. There is thus a need for a signal quality indicator that overcomes these problems.

SUMMARY OF THE INVENTION

It is an object of some aspects of the present invention to detect and correct an error in a word received in a data receiver.

In preferred embodiments of the present invention, an input sequence of words is received in a data receiver. The receiver analyses the words, and for each received word the receiver forms candidate words corresponding to the received word, and assigns a respective probability of being correct to each candidate word. The probabilities are determined in response to a measured grade of at least one of the bits of the received word. The candidate words are inserted, on a continuing basis as the sequence of data words is received, into heads of candidate word sequences stored in a memory of the receiver. The receiver maintains a status of one of the candidate sequences as a most-preferred candidate sequence, i.e. the candidate sequence having the highest probability of having correct words. The other one or more candidate sequences have the status of less-preferred candidate sequences. The receiver analyses the candidate sequences, and for each candidate sequence determines a candidate sequence parameter indicative of the existence of an erroneous word within the candidate sequence.

The receiver grades the candidate words formed on the basis of how well the candidate words “fit” the candidate sequences. The grade is estimated responsive to the candidate sequence parameters as they exist, the probabilities of the possible words, and how the candidate sequence parameters would change as each word or candidate word is inserted into the candidate sequence. Responsive to the grade, the receiver may change the status of the candidate sequences, and then insert the candidate words into the sequences, so as to maintain the status of one of the candidate sequences as the most-preferred candidate sequence. The receiver then outputs a word from the end of the most-preferred candidate sequence. By constantly updating the status of the candidate sequences as new words are input to the receiver, the receiver is able to generate a most-preferred candidate sequence having a high probability of having no erroneous words, so that the system acts to effectively correct words having errors.

In some preferred embodiments of the present invention, the words are encoded according to an 8b/10b standard coding system which incorporates a disparity check. In this case each candidate sequence parameter is most preferably the running disparity of the sequence of words which have been entered into the candidate sequence, and the grade for each candidate word is estimated on the basis of the disparity of the candidate word and the running disparity of the candidate sequence into which the candidate word is to be inserted.

There is therefore provided, according to a preferred embodiment of the present invention, a method for processing an input sequence of words received over a communication link, including:

assigning a respective bit-grade to at least one of the bits in a first word among the words in the input sequence;

deriving candidate words from the first word in response to the respective bit-grade;

inserting one of the candidate words into each of a plurality of candidate sequences, so that each of the candidate sequences contains one of the candidate words;

adding one or more subsequent words to each of the candidate sequences, each of the subsequent words consisting of a further candidate word derived from a further word received in the input sequence;

computing respective sequence parameters for the candidate sequences, based on a relation between the candidate words and the subsequent words in each of the candidate sequences;

selecting one of the candidate sequences in response to the sequence parameters; and

outputting the one of the candidate words contained in the selected one of the candidate sequences.

The method preferably also includes assigning a respective word-grade to the one or more subsequent words in response to the one or more subsequent words and the respective sequence parameters, and adding the one or more subsequent words includes adding the one or more subsequent words to each of the candidate sequences in response to the respective grade.

Preferably, the respective word-grade is operative as an index in a predetermined lookup table containing entries for adding the one or more subsequent words.

The method preferably includes assigning a respective word-grade to the one or more subsequent words in response to the one or more subsequent words and the respective sequence parameters, wherein the plurality of candidate sequences includes a first candidate sequence and a second candidate sequence, and copying the first candidate sequence to the second candidate sequence in response to the respective grade.

Preferably, the respective word-grade is operative as an index in a predetermined lookup table having entries for copying the first candidate sequence to the second candidate sequence.

Preferably, selecting the one of the candidate sequences includes assigning the one of the candidate sequences a highest probability of containing correct words.

Preferably, the at least one of the bits includes all the bits in the first word, and assigning the respective bit-grade comprises determining respective higher probability values and lower probability values for all the bits, and wherein the candidate words comprise a first candidate word having all the higher probability values and a second candidate word having all the lower probability values.

Preferably, the first and the second candidate words include respective first and second disparities, wherein computing the respective sequence parameters includes computing respective running disparities for the candidate sequences, and wherein computing the respective sequence parameters includes comparing the first and second disparities and the running disparities.

Further preferably, the first and the second candidate words include 10-bit words which are encoded from respective 8-bit words according to a predetermined standard.

There is further provided, according to a preferred embodiment of the present invention, apparatus for processing an input sequence of words received over a communication link, including:

a processor which is adapted to:

assign a respective bit-grade to at least one of the bits in a first word among the words in the input sequence, and

derive candidate words from the first word in response to the respective bit-grade; and

a memory, which stores a plurality of candidate sequences, so that each of the candidate sequences contains one of the candidate words, the processor being further adapted to:

insert one of the candidate words into each of the plurality of candidate sequences,

add one or more subsequent words to each of the candidate sequences, each of the subsequent words consisting of a further candidate word derived from a further word received in the input sequence,

compute respective sequence parameters for the candidate sequences, based on a relation between the candidate words and the subsequent words in each of the candidate sequences,

select one of the candidate sequences responsively to the sequence parameters, and

output the one of the candidate words contained in the selected one of the candidate sequences.

Preferably, the processor is further adapted to assign a respective word-grade to the one or more subsequent words responsively to the one or more subsequent words and the respective sequence parameters, and to add the one or more subsequent words to each of the candidate sequences responsively to the respective word-grade.

Preferably, the respective word-grade is operative as an index in a predetermined lookup table containing entries for adding the one or more subsequent words, and the memory stores the lookup table.

Further preferably, the processor is further adapted to assign a respective word-grade to the one or more subsequent words responsively to the one or more subsequent words and the respective sequence parameters, wherein the plurality of candidate sequences includes a first candidate sequence and a second candidate sequence, and wherein the processor is further adapted to copy the first candidate sequence to the second candidate sequence responsively to the respective word-grade.

Preferably, the respective word-grade is operative as an index in a predetermined lookup table having entries for copying the first candidate sequence to the second candidate sequence, and the memory stores the lookup table.

Preferably, the processor is adapted to assign the one of the candidate sequences a highest probability of containing correct words.

Further preferably, the at least one of the bits includes all the bits in the first word, and the processor is adapted to determine respective higher probability values and lower probability values for all the bits, and the candidate words include a first candidate word having all the higher probability values and a second candidate word having all the lower probability values.

Preferably, the first and the second candidate words have respective first and second disparities, and the processor is adapted to compute respective running disparities for the candidate sequences and to compare the first and second disparities and the running disparities.

Preferably, the first and the second candidate words include 10-bit words which are encoded from respective 8-bit words according to a predetermined standard.

There is further provided, according to a preferred embodiment of the present invention, a method for processing an input sequence of words received over a communication link, including:

identifying a possible bit error in a first word among the words in the input sequence;

deriving candidate words from the first word responsively to the possible bit error;

inserting one of the candidate words into each of a plurality of candidate sequences, so that each of the candidate sequences contains one of the candidate words;

adding one or more subsequent words to each of the candidate sequences, each of the subsequent words consisting of either a further word received in the input sequence subsequent to the first word, or a further candidate word derived from the further word received in the input sequence;

computing respective sequence parameters for the candidate sequences, based on a relation between the candidate words and the subsequent words in each of the candidate sequences;

selecting one of the candidate sequences responsively to the sequence parameters; and

outputting the one of the candidate words contained in the selected one of the candidate sequences.

There is further provided, according to a preferred embodiment of the present invention, apparatus for processing an input sequence of words received over a communication link, comprising:

a processor which is adapted to:

assign a respective quality-measure to at least one of the bits in a first word among the words in the input sequence, and

derive candidate words from the first word in response to the respective quality-measure; and

a memory, which stores a plurality of candidate sequences, so that each of the candidate sequences contains one of the candidate words, the processor being further adapted to:

insert one of the candidate words into each of the plurality of candidate sequences,

add one or more subsequent words to each of the candidate sequences, each of the subsequent words consisting of a further candidate word derived from a further word received in the input sequence,

compute respective sequence parameters for the candidate sequences, based on a relation between the candidate words and the subsequent words in each of the candidate sequences,

select one of the candidate sequences responsively to the sequence parameters, and

output the one of the candidate words contained in the selected one of the candidate sequences.

The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a deserializer, according to a preferred embodiment of the present invention;

FIG. 2 are schematic graphs of data received by the deserializer of FIG. 1, according to a preferred embodiment of the present invention;

FIG. 3 is a schematic block diagram of an initial grading module, according to a preferred embodiment of the present invention;

FIG. 4 is a schematic block diagram of a leakage integrator, according to a preferred embodiment of the present invention;

FIG. 5 is a schematic block diagram of a single bit corrector, according to a preferred embodiment of the present invention;

FIG. 6 is a schematic block diagram illustrating an error correction system, according to a preferred embodiment of the present invention;

FIG. 7 is a logical flow diagram which schematically illustrates a process carried out by the error correction system of FIG. 6, according to a preferred embodiment of the present invention;

FIG. 8 is a flowchart showing steps in the process of FIG. 7, according to a preferred embodiment of the present invention;

FIG. 9 is a schematic block diagram of a signal quality indicator (SQI), according to a preferred embodiment of the present invention;

FIG. 10 is a schematic block diagram of leakage integrators, according to a preferred embodiment of the present invention;

FIG. 11 shows schematic graphs of values of the final signal quality grade from the SQI of FIG. 9, for different input signals, according to a preferred embodiment of the present invention; and

FIG. 12 is a schematic block diagram of a multi-channel deserializer, according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1, which is a schematic block diagram of a deserializer 10, and to FIG. 2, which comprises schematic graphs of data received by the deserializer, according to a preferred embodiment of the present invention. In an analog front end 11, deserializer 10 receives incoming serial data which may be transmitted according to substantially any serial data protocol. Hereinbelow, by way of example, the data is assumed to be transmitted in the form of 8b/10b encoded data according to IEEE standard 802.3z, as described in the Background of the Invention. The data is received on a channel, herein assumed to comprise two differential lines 12, although it will be understood that the channel may not comprise differential lines.

The data is combined in an input cell 13 as a single data stream 50 of bits 54, as shown in a graph 52 (FIG. 2), and the single bit stream is fed to a sample generator 20. Bits 54 are also referred to herein as bits B1, B2, . . . , B10. A section of graph 52 is shown in more detail in a graph 56. Data stream 50 is assumed to be transmitted at 3.125 Gb/s, so that each bit 54 of stream 50 has a nominal width of 320 ps. However, it will be appreciated that the transmission rate and nominal width are examples, and that substantially any transmission rate and bit width may apply to the data received.

A free-running reference receive clock 14 driving a phase-locked loop (PLL) oscillator 16 generates a base frequency of 625 MHz. The 625 MHz base frequency is used to generate 20 substantially equally spaced phases, ph0, ph1, . . . , ph19 which are separated by 80 ps. The phases are input to a multiplexer 18, and contiguous phases from the multiplexer are used to sample bits 54 in sample generator 20. Sample generator 20 effectively acts as a slicer, providing a decision of 0 or 1 at each sample point.

As shown in graph 52, the 20 phases are used to sample a first set of five bits {B1, B2, B3, B4, B5}, and are also used to sample a second set of five bits {B6, B7, B5, B9, B10}, each bit being nominally sampled at four positions. Generator 20 thus generates a total of forty samples in a cycle defined by the ten bits. The samples are provided in the form of respective decisions which are transferred to a digital circuitry section 22, which also receives general timing signals derived from clock 14 and/or PLL oscillator 16. It will be appreciated that the separation of 80 ps is a fourth of the period of the nominal width. It will also be appreciated that the separation of 80 ps is chosen by way of example, and that the phases may be separated by substantially any integral sub-multiple of the nominal width, the number of decisions generated by generator 20 altering accordingly.

In digital circuitry 22 the forty decisions are grouped into four sampling sets A, B, C, D. Referring to FIG. 2, first set A comprises ten decisions—two decisions for each phase—generated by phases {ph0, ph4, ph8, ph12, ph16}. Sets B, C, and D respectively comprise ten decisions having phases {ph1, ph5, ph9, ph13, ph17}, {ph2, ph6, ph10, ph14, ph18}, and {ph3, ph7, ph11, ph15, ph19}. Each sampling set is fed through one of four substantially similar initial grading modules 24. Each module 24 determines a quality of its respective sample set as a temporal grade, by comparing values of a present decision with values of adjacent decisions. The initial grades generated in each module 24 are integrated in respective leakage integrators 26, and the integrated grades are used in a main phase selector 28, as is described in more detail hereinbelow, to determine an optimal sampling set from amongst sampling sets A, B, C, D. Both the integrated grades supplied to main phase selector 28, and a grade determined by the selector, are thus determined by averaging decisions of more than one phase or phase set.

The optimal sampling set, together with the original decisions, are processed in a single bit corrector 32 wherein errors that may be caused by a “high frequency” single bit occurring within a “low frequency” pattern are eliminated. Bits from corrector 32 are processed through a symbol alignment block 34, wherein symbols input to deserializer 10 are recovered. Symbols from deserializer 10 are preferably output via an error correction block 150. Corrector 32 and blocks 34 and 150 are also described in more detail below. Most preferably, main phase selector 28 also provides outputs which are used as inputs to a signal quality indicator 27, preferably comprised in deserializer 10, and described with reference to FIGS. 8, 9, and 10 below.

FIG. 3 is a schematic block diagram of one of initial grading modules 24, according to a preferred embodiment of the present invention. Each module 24 operates in parallel on the ten decisions of its sampling set, so that elements 60, 62, 68, 70, 72, 74, and 76, in the module are replicated ten times. Elements 60 and 62 respectively comprise comparators, and are herein referred to as comparators 60 and 62; elements 68 and 70 respectively comprise XOR gates, and are herein referred to as gates 68 and 70; elements 72 and 74 respectively comprise AND gates, and are herein referred to as gates 72 and 74; element 76 comprises a summer and is herein referred to as summer 76.

Comparator 60 compares a decision value D(p,n), for a bit n, of the present phase p with a decision value D(p−1,n) of a phase prior to the present phase. The output of comparator 60 is a first input to AND gate 72. Comparator 62 compares decision value D(p,n) of the present phase with a decision value D(p+1,n) of a phase after the present phase. The output of comparator 62 is a first input to AND gate 74.

Module 24 also comprises selectors 64 and 66, which receive 12 decision values D(M) of a main phase M. Generation of main phase M is described in more detail below. Selector 64 selects ten decision values D(M,n+1), corresponding to main phase decisions of a bit after bit n, and outputs the selected decisions as a first input of XOR gate 68. Selector 66 selects ten decision values D(M,n−1), corresponding to main phase decisions of a bit before bit n, and outputs the selected decisions as a first input of XOR gate 70. The result of gate 68 provides a second input to gate 74, and the result of gate 70 provides a second input to gate 72.

The respective outputs of gates 72 and 74 are summed in summers 76. Summers 76 thus output ten separate values, herein termed partial sums PSn, for each of the ten bits considered in stream 50. The ten values PSn are summed in a second summer 78 to give one value, which is delayed in a delay 80 before outputting a temporal grade TG(p) for present phase p from initial grading module 24.

The output of each module 24 may be represented by the following equation:

Each module 24 compares sample decisions of three consecutive bits, (n−1, n, n+1, where n=1, . . . , 10). The ten results of these comparisons are summed, as shown by equation (1), in order to grade each of the sampling phase sets.

Graph 56 illustrates the summation. In graph 56 bits B4, B5, and B6 are respectively assumed to have the values 0, 1, and 0, and n has the value 5.

Each initial grading module 24 evaluates ten partial sums PSn, by using a total of forty samples from the ten bits being processed by the modules. As shown by equation (1), the evaluation compares values generated by three consecutive bits (n−1, n, n+1). It will be appreciated that in order to evaluate the first bit (n=1) of a specific group of ten bits, values for the tenth bit of the preceding group are required for the evaluation. Similarly, to evaluate the tenth bit of the specific group, values for the first bit of the following group are required for the evaluation. The total of 48 samples of the twelve bits are stored in multiplexers 64 and 66.

Performing similar calculations to equations (2)-(5) for all bits and assuming the bits alternate sequentially in value between 0 and 1, gives results for TG(p) as shown in Table II below.

TABLE II

Phase p

TG(p)

A

10

B

20

C

20

D

10

Temporal grades TG(p) form a basis for deserializer 10 to decide which sampling phases to use in evaluating bits {B1, . . . , B10}. As is apparent from Table II, phases which are closer to transitions between values, i.e., phase A and D in the table, receive substantially lower grades than phases which are farther from the transitions, i.e., phases B and C. The calculations of grades thus enable the deserializer to select a sampling phase furthest from transitions between values. The selected sampling phase, also herein termed the main phase, is used by the deserializer as a decoding phase, i.e., as an optimal phase at which bits 54 are to be decoded.

It will be understood that while the examples above with reference to the graphs of FIG. 2 have used substantially ideal values, the principles of grading incoming bits as described hereinabove apply to non-ideal received bits. In the case of non-ideal bits, deserializer 10 continuously grades the bits and determines a highest grade G from amongst three adjacent phases, as is shown in equation (7) below. Except when there is a change in phase, it will be appreciated that the highest grade phase, i.e. the main phase, will be the “center” of the three graded phases. When there is a change in main phase, then for one cycle the highest graded phase will be one of the non-central graded phases.

The resultant TG(p) of each initial grading module 24 is integrated in a respective leakage integrator 26.

FIG. 4 is a schematic block diagram of leakage integrator 26, according to a preferred embodiment of the present invention. Each integrator 26 performs a weighted time integration of the value TG(p) received from its respective initial grading module 24. TG(p) is input to a shifter 90, which shifts the value of TG(p) to the right by a predetermined number, preferably 2. The output of shifter 90 is a first input to a summer 92. The output of summer 92 is passed through a register 96 acting as a time delay, and the output of register 96 is fed back directly to the summer. The output of register 96, after being shifted right by the predetermined number in a shifter 94, is also subtracted in summer 92. The output from the integrator, after being adjusted in a fixed point converter 98, is represented by the following equation:
G(p,t)=G(p,t−1)−G(p,t−1)>>a+TG(p)>>a (6)

where G(p,t) is the final grade of phase p at a time t, and

a is the predetermined shifted right value.

Each of the four final grades is input to a main phase selector 28 (FIG. 1). In each cycle of the 625 MHz clock selector 28 selects a main phase M(t+1) for a next cycle by finding a highest grade G from three adjacent phases of the present cycle, as shown in the following equation:
M(t+1)=Max[G(M,t),G(M−1,t),G(M+1,t)] (7)

The selected main phase M(t+1) is used, as shown in equation (1), as an input for determining the partial sums PSn. Preferably, if there is no clear-cut maximum in equation (7), G(M,t) is assumed to be the maximum value.

Returning to FIG. 1, an index D1 of main phase M and an index D2 of a second phase, the second phase having a grade closest to main phase M, are transferred from main phase selector 28 to single bit corrector 32. Corrector 32 also receives decisions from sample generator 20, via a delay 30. Corrector 32 uses the phase indices and decisions corresponding to main phase M to allow a decision made by the main phase to be overwritten in predetermined situations, usually caused by inter-symbol interference (ISI). Typically, ISI is most troublesome when a single bit value is different from a train of bits on either side of the single bit, for example 1111110111. Most preferably, a main phase decision is overwritten if the following condition is true:
((D(M,n−1)=D(M,n)=(D(M,n+1))⊕(D(p,n)≠D(M,n)) (8)

where p may be M−1 or M+1.

Condition (8) is true if three consecutive main phase decisions are the same, and if the central main phase decision is not the same as a phase on either side of the central main phase. The latter typically occurs if the main phase “missed” a transition. If condition (8) is not true, the decision of the main phase is not overwritten.

FIG. 5 is a schematic block diagram of single bit corrector 32, according to a preferred: embodiment of the present invention. Correctors substantially similar to single bit corrector 32 are most preferably implemented in parallel, the number of correctors preferably corresponding to ten. Corrector 32 comprises a comparator 100 which checks for equality of decisions D(M,n−1), D(M,n), and D(M,n+1). The decisions are received via delay 30. The output of comparator 100 is a first input to an AND gate 108. Two other substantially similar comparators 102, 104 check respectively for inequality of decisions D(M−1,n) and D(M,n), and decisions D(M+1,n) and D(M,n), which are also received via delay 30. D(M,n) corresponds to main phase index D1, and either D(M+1,n) or D(M−1,n) correspond to second phase index D2. The decision D(M+1,n) or D(M−1,n) which does not correspond to D2 represents a third phase decision, on the opposite side of the main phase from D2. The outputs of comparators 102 and 104 are transferred to an OR gate 106, which generates a second input to AND gate 108. The output of AND gate 108, corresponding to equation (8), is exclusively ORed in a gate 110 to decide if main decision D(M,n) is to be overwritten.

It will be appreciated that since the frequency of receiver clock 14 and the effective frequency of the received bits may not be identical, there may typically be drift between the sampling positions generated by the clock and the received bits. Typically, there is a standard number of bits resolved per cycle, the standard in the examples described above being ten; the drift will cause, for one cycle, typically the cycle when there is a change in main phase, resolution of one extra bit or one less bit in the cycle compared to the standard number of bits resolved. Thus corrector 32 may output, in each cycle of the receiver clock, 9, 10, or 11 bits.

Decisions from corrector 32 for phase index D1, as well as decisions for phase D2, are transferred to symbol alignment block 34 which temporarily stores the decisions as sets of D1 decisions and sets of D2 decisions. Bits 52 are preferably transmitted as symbols, also termed words, formed according to a predetermined coding scheme, most preferably the 8b/10b word coding scheme described in the Background of the Invention. Block 34 analyzes the stored values to determine boundaries between symbols, by methods which are well known in the art, and outputs the symbols evaluated. Typically one symbol formed from the D1 decisions, herein termed W1, is output per cycle, but it will be appreciated that in a generally similar manner to corrector 32 outputting one extra or one less bit per cycle, alignment block 34 may be able to resolve and output 0, 1 , or 2 symbols per cycle. A second symbol, formed from the D2 decisions and herein termed W2, is also output from block 34. Symbols W1 and W2 are also termed candidate words hereinbelow. It will be appreciated that, since its bits are derived from main phase decisions, W1 has a significantly higher probability of being correct than W2, which is derived from second phase decisions. The property of the difference in probability, generated by assigning a main phase and a second phase for each bit, is used in error correction block 150.

As outlined in the Background of the Invention, encoding 8b words to 10b words enables errors in reception of the 10b words to be detected.

Table III below shows how the errors introduced by an incorrect single bit in the 10b word may be classified.

TABLE III

No.

Class Description

1

The 10b word is invalid i.e., it is not present in mapping B1 or B2,

Table I.

2

The 10b word belongs to an incorrect mapping, according to the

disparity status of a string of 10b words already received.

3

The 10b word belongs to a correct mapping, but causes the string

to expect a disparity switch when no switch should occur.

4

The 10b word belongs to a correct mapping, but causes the string

not to expect a disparity switch when such a switch should occur.

Block 34 is most preferably implemented so as not to output 10b words in classification 1. Preferred embodiments of the present invention are implemented to correct errors in classifications 2, 3, and 4 , as described hereinbelow.

FIG. 6 is a schematic block diagram illustrating an error correction system, according to a preferred embodiment of the present invention. W1 and W2 are input to error correction block 150, which recovers errors in the 8b/10b words it receives. It will be appreciated that block 150 may be implemented to recover errors generated by transmission of other types of encoded signals which have redundancy.

Block 150 maintains a multiplicity of sequences of previously transmitted candidate words Wx(t), Wx(t−1), . . . , Wx(t−N+1), where x may be 1 or 2, and where N is the number of words W1, W2 comprised in each sequence. The sequences are stored in a memory 152 in correction block 150. Herein, by way of example, the number of sequences is assumed to be three, and the sequences, also herein termed tracks, are referred to as T1, T2, and T3. Except as described below, track T1 in general receives W1, and track T2 in general receives W2. T1 is assumed to be a preferred track, and gives a final output from block 150. T2 is assumed to be a less preferred track. Track T3 is used as a reserve track.

A processor 154 in block 150 calculates a running disparity (RD) of each sequence, determining if the disparity status is positive, zero, or negative. Disparity and running disparity, and the concept of a transmitter generating strings of 10b words having their RD maintained within bounds, are described in more detail in the Background of the Invention.

As shown in Table III, errors may be classified as class 2, 3, or 4. A class 2 error is immediately apparent, assuming there are no prior errors in the string to which the word is being inserted. A class 3 or 4 error may not be immediately apparent, but eventually causes a disparity error similar to class 2. Processor 154 accommodates the differing errors by copying tracks T1, T2, and T3 to each other, and by assigning W1 and W2 to the tracks, so as to maintain T1 as the preferred track with the highest probability of having correct words in the track.

As candidate words W1 and W2 are generated, processor 154 checks if the candidate words “fit” the sequences, updates the sequences, and inserts W1 and W2 into the updated sequences according to the most probably correct arrangement.

For example, if W1 fits T1, W2 fits T2, but neither fit T3, T1 is first copied to T3 since T1 is more probably correct than T2. W1 is then inserted to T1 and T3, and W2 is inserted to T2. If W1 or W2 fit T3, no tracks are copied, W1 is inserted to T1, W2 is inserted to T1, and either W1 or W2 is inserted to D3, depending which of W1, W2 fits T3. If both W1, W2 fit T3, then W1 is inserted to T3, since W1 is more probably correct than W2. (This example is also considered with reference to Table V below.)

FIG. 7 is a schematic diagram illustrating stages in a process 160 operated by error correction block 150, and FIG. 8 is a flowchart for the process, according to a preferred embodiment of the present invention. Process 160 is applied by processor 154 to each candidate word as it is received from symbol alignment block 34.

In a first step 162 of the process, corresponding to a first stage 161, processor 154 receives the two possible candidate words W1 and W2. Except for the case of W1=W2 words W1 and W2 may differ by one or more bits, the probability of a specific number of bits difference decreasing as the number increases. In most cases of a difference existing, the difference is one bit. Examples of possible pairs of words differing by one bit (derived from Table I) are given in Table IV below. The “difference” bit is underlined for each 10b word.

TABLE IV

Position in Table I

W1/W2

Decimal 0, Second mapping

011000 1011

Decimal 6, First mapping

011001 1011

Decimal 188, First mapping

001110 1010

Decimal 189, First mapping

101110 1010

Decimal 196, Second mapping

001010 0110

Decimal 228, Second mapping

001010 1110

In a second step 164, processor 154 utilizes Table I, stored in memory 152, to determine to which mapping, B1 or B2, each word W1 and W2 belongs.

In a third step 166, for each W1, W2 word received in step 162, processor 154 determines a respective grade G1, G2. The grade is an ordered triple (Fit T3, Fit T2, Fit T1), each element of the triple comprising a binary value of 0 or 1. A “1” indicates a “fit,” i.e., that the word may be inserted into the respective track T1, T2, or T3, without an error being apparent in the updated track. A “0” indicates a “no-fit,” i.e., that inserting the word would generate an error in the track. For example a grade (0,1,1) assigned to W1 means that W1 does not fit track T3, but does fit tracks T2 and T1. If W1=W2 then G2 is automatically allocated the value (0,0,0).

It will be appreciated that a fit does not necessarily mean that a sequence with the inserted word has no erroneous words. A sequence after the word has been inserted may comprise a “hidden” error corresponding to a category 3 or 4 error. The error may be in the inserted word, or in a word further back in the sequence. Similarly, a no-fit does not necessarily mean that the word being inserted has an error. The no-fit may also be the result of a sequence having a hidden category 3 or 4 error.

In a fourth step 168, corresponding to a second stage 163, processor 154 uses Table V below to assign which tracks replace each other, and also into which tracks words W1 and W2 are inserted. Table V is stored in memory 152. In the table T1→T2 means that track T1 is copied to track T2, W1→T1 means that W1 is inserted to the head of track T1. It will be understood that entries in the table such as

T⁢⁢1→T⁢⁢2T⁢⁢2→T⁢⁢1
mean that the track initially labeled T1 is copied to T2, and the track initially labeled T2 is copied to T1, so that in this case the tracks essentially switch labels. Processor 154 copies the tracks, with their running disparity, as indicated in the table.

TABLE V

G2

G1

000

001

010

011

100

101

110

111

000

T1→T2

T2→T1

T1→T3

T3→T1

T1→T2

T2→T1

T1→T3

T2→T3

T3→T2

W1→T1

W2→T1

W2→T1

W2→T1

W2→T1

W2→T1

W2→T1

W2→T1

W1→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W1→T3

W2→T3

W2→T3

W2→T3

W2→T3

W2→T3

W2→T3

W2→T3

001

T1→T2

T1→T2

T1→T3

T1→T3

T3→T2

T1→T2

T1→T3

T1→T3

T1→T3

T1→T3

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W1→T3

W1→T3

W1→T3

W2→T3

W1→T3

W2→T3

W2→T3

W2→T3

010

T2→T1

T1→T2

T2→T1

T2→T1

T3→T2

T2→T1

T2→T1

T2→T1

T2→T3

T2→T1

T2→T3

T1→T3

T2→T1

T1→T2

T1→T3

T2→T3

T2→T3

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W1→T3

W1→T3

W1→T3

W2→T3

W1→T3

W2→T3

W2→T3

W2→T3

011

T1→T3

T1→T2

T2→T3

T2→T3

T2→T3

T1→T2

T2→T3

T2→T3

T2→T3

T3→T2

T2→T3

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W1→T3

W1→T3

W1→T3

W1→T3

W1→T3

W1→T3

W1→T3

W1→T3

100

T3→T1

T1→T2

T3→T1

T1→T3

T3→T1

T1→T2

T3→T1

T3→T1

T3→T2

T3→T1

T3→T1

T3→T2

T3→T1

T1→T3

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W1→T3

W1→T3

W1→T3

W2→T3

W1→T3

W2→T3

W2→T3

W2→T3

101

T1→T2

T1→T2

T1→T3

T3→T2

T1→T2

T1→T3

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W1→T3

W1→T3

W1→T3

W2→T3

W1→T3

W1→T3

W1→T3

W2→T3

110

T2→T1

T1→T2

T2→T1

T1→T3

T2→T1

T2→T1

T2→T1

T2→T1

T2→T1

T3→T2

T1→T3

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W1→T3

W1→T3

W1→T3

W2→T3

W1→T3

W1→T3

W1→T3

W2→T3

111

T1→T2

T2→T3

T2→T3

T2→T3

T2→T3

T2→T3

T2→T3

T2→T3

T3→T2

T1→T2

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T1

W1→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W2→T2

W1→T3

W1→T3

W1→T3

W1→T3

W1→T3

W1→T3

W1→T3

W1→T3

It will be understood that the operations listed in Table V are based on maintaining track T1 as the track being most likely to comprise a correct string of received words. To illustrate the operations listed in Table V, consider the example described above, which corresponds to a set of four ordered pairs of triples: {(G1, G2) }={((0,0,1),(0,1,0)); ((1,0,1),(0,1,0)); ((0,0,1),(1,1,0)); ((1,0,1),(1,1,0))}. Inspection of the four cells of Table V corresponding to the ordered pairs shows that the actions carried out correspond to those described above in the example. Actions listed for other cells of Table V are generated in a generally similar manner as those described for the cells of the example.

In a final step 170, processor 152 outputs as a final decision the word that is in track T1.

It will be understood that the principles of the present invention may be applied to correcting erroneous words which have been encoded in formats other than the 8b/10b format described above, or that may not be encoded, and for correcting errors in words which have one or more incorrect bits. It will be further understood that while the preferred embodiments described above use three sequences of stored words, other numbers of sequences may also be used. For example, the number of sequences may be set to four, so that there are two reserve sequences, each of which may have either word W1 or W2 inserted. All such numbers are assumed to be comprised within the scope of the present invention.

In an alternative preferred embodiment of the present invention error, correction block 150 does not receive two words W1 and W2 from symbol alignment block 34. Rather block 150 receives one word, preferably W1, from block 34 and a single bit quality value Q (FIG. 6). Most preferably, the single bit quality value is in the form of a flag assigned to a specific bit in the word received by block 150 whose quality has been assessed and which is considered to be problematic on the basis of the assessment. It will be appreciated that in performing their tasks, both selector 28 and corrector 32 are able to generate a measure of the quality of each single bit they analyze. For example, if single bit corrector 32 does perform a correction using condition (8), the bit value output from the corrector may be considered to have a high probability of being correct. Thus the bit quality of the bit value output is high, and correspondingly, a bit quality for the opposite bit value for this bit is low. Those skilled in the art will be able to assign a bit quality for bits output from selector 28.

Preferably, if the bit quality is outside a predetermined value, so indicating that the bit may not be correct and that the bit is problematic, bit quality Q is input to error correction block 150, most preferably by setting the flag if implemented. If the bit quality is within the predetermined value, so that the corresponding bit is assumed to be correct, no bit quality value is input to block 150, and the flag is not set.

Block 150 uses the bit quality and the symbol associated with the bit to construct a second word W2. Process 160, as described above with respect to FIGS. 7 and 8, is then applied to W1 and W2.

As stated above, since clock 14 is not locked to a transmit clock of the incoming signal, the sampling positions of sampling phase sets A, B, C, D, (FIG. 2) may drift relative to data stream 50. As the positions drift, they effectively scan across the data stream. The scanning, and the fact that a single value of G(p,t) (equation (6)) acts as a weighted average of signal levels at three adjacent phases, are used by preferred embodiments of the present invention to implement a signal quality indicator 27 (FIG. 1). The signal quality indicator may be advantageously used in place of specialized signal quality measurement equipment such as that described in the Background of the Invention. Signal quality indicator 27 receives its inputs, an index D1 of the main phase and a grade G(p,t) of that phase, from main phase selector 28.

FIG. 9 is a schematic block diagram of signal quality indicator (SQI) 27, according to a preferred embodiment of the present invention. SQI 27 comprises a first leakage integrator 180 in series with a second leakage integrator 182. Both integrators integrate their respective inputs so as to effectively smooth them. Leakage integrator 182 may be activated by an enable signal generated by a multiplexer 184, so that in addition to integrating its input, decimation may be performed on the output of SQI 27. The enable signal for the decimation is derived from a multiplexer 184, which activates the enable signal according to a decimation factor received by the multiplexer.

The decimation factor is most preferably generated automatically by a drift estimation block 186. Block 186 receives, from main phase selector 28, the phase value that has been selected as the main phase. Block 186 also receives a timing signal, preferably generated from clock 14, which enables the block to determine a duration of time for which a specific phase is the main phase. During operation of deserializer 10 the main phase changes because of drift of the sampling phase sets, as described above. Block 186 measures a “phase time” during which a specific phase of the sampling phase sets is chosen as the main phase. The measured phase time is approximately inversely proportional to a “drift speed” of the sampling phases on the data stream.

In order to scan across the data stream at a rate which is approximately independent of drift speed, block 186 preferably sets the decimation factor to be approximately inversely proportional to the drift speed, so that the lower the drift speed the higher the decimation factor. Block 186 thus preferably sets the decimation factor to be directly proportional to the phase time. In a preferred example of the present invention, Block 186 automatically sets the decimation factor so that eight samples are taken from a specific phase, i.e., during the phase time. Optionally, multiplexer 184 may also receive an alternative decimation factor, which may be input directly to the multiplexer from an operator of SQI 27. Such an operator input may be used, for example, in a case where the drift speed is very low or even substantially zero.

FIG. 10 is a schematic block diagram of leakage integrators 180, 182, according to a preferred embodiment of the present invention. Apart from the differences described below, the operation of integrators 180 and 182 is generally similar to that of integrator 26 (FIG. 4), so that elements indicated by the same reference numerals in integrators 26, 180, and 182 are generally identical in construction and in operation. In integrators 180 and 182, shifters 90 and 94 preferably shift their input to the right by 6, the value effectively controlling the size of a “sliding window” over which samples are integrated. Unlike integrator 26, neither of integrators 180, 182 have a fixed point converter 98 at their output. Integrator 182 also receives an enable input to shifter 90, so that the shifter is activated according to the decimation factor used by multiplexer 184, and so that integrator 182 performs its integration only when enabled. Integrator 182 outputs a final signal quality grade.

The combination of two integrators in series, the second having decimation, gives sufficient averaging to substantially eliminate noise effects and also effectively scan across each bit of the incoming data. The two integrators give more flexible and better control of integration parameters, as well as using less hardware than an equivalent single integrator providing the same functions as the two integrators.

Measurements of the signal quality grade are preferably made on incoming signals having the same data, for example, random idle signals. Such measurements on the same data may be performed, for example, during initial setup and adjustment of deserializer 10 and its incoming lines, when a remote transmitter may be requested to transmit specific data.

FIG. 11 shows schematic graphs of values of the final signal quality grade for different input signals, according to a preferred embodiment of the present invention. Five different input signals were simulated and input to deserializer 10. The five signals had different qualities, as determined by an eye opening measurement based on the system described in the Background of the Invention. Graphs 202, 204, 206, 208, and 210 show values of the signal quality grade, as measured by SQI 27, vs. time. It is seen that the grades for each input signal stabilize to a substantially constant value. Furthermore, the values obtained are substantially independent of the rate of decimation introduced in integrator 182, and of the drift speed, even when the latter is very low or substantially zero. Table VI shows the stabilized grade values, from SQI 27 for the different input signals, together with the eye opening measurement for the signals.

TABLE VI

Graph

Stabilized Signal Quality Grade

Eye Opening Value

202

0.47

0.427

204

0.64

0.490

206

0.65

0.494

208

0.72

0.525

210

0.72

0.526

Graph 212 plots the stabilized signal quality grades vs. the eye opening values. It is seen both from Table VI and from graph 212 that there is a substantially linear relationship between the signal quality grades and the eye opening values, so that the grades provide a good metric of the signal quality.

FIG. 12 is a schematic block diagram of a multi-channel deserializer 230, according to a preferred embodiment of the present invention. Multi-channel deserializer 230 comprises a plurality of separate deserializers 232. Apart from the differences described below, the operation of each deserializer 232 is generally similar to that of deserializer 10, so that elements indicated by the same reference numerals in both deserializers 10 and 232 are generally identical in construction and in operation. Preferably, none of analog sections 11 of deserializers 232 have clock 14, PLL oscillator 16, or multiplexer 18. Rather multi-channel deserializer 230 comprises a phase generation block 234, comprising a single clock 264, a PLL oscillator 256, and a multiplexer 268, respectively substantially similar to clock 14, PLL oscillator 16, and multiplexer 18. Block 234 provides twenty phases ph0, ph19, substantially as described above with reference to FIG. 1, to each of sample generators 20 in deserializers 232, and general timing signals to each of their digital circuitry 22. Alternatively, instead of phase generation block 234, one of analog sections 11 in a specific deserializer 232 comprises single clock 264, PLL oscillator 256, and multiplexer 268, which generate the twenty phases and general timing signals for the deserializer, and which provide the twenty phases and timing signals to the other analog sections 11 and digital circuitry sections 22 respectively of the other deserializers 232.

Each deserializer 232 receives a channel A, B, C, . . . of data, and de-serializes its respective data stream substantially as described above for deserializer 10. It will be appreciated that multi-channel deserializer 230 is able to deserialize substantially any number of channels of incoming serial data, one deserializer 232 for each channel, using only one PLL clock. Multi-channel deserializer 230 thus saves significant numbers of components, as well as significantly reducing the complexity and difficulty of their arrangement, compared to multi-channel deserializers comprising more than one PLL clock, typically one per channel plus a synchronizing PLL clock, and which may also require elastic buffers. It will be appreciated that multi-channel deserializers such as deserializer 230, when implemented on a single die, have significant improvements in yields compared to deserializers having multiple PLL clocks, since any single PLL failure leads to failure of the whole deserializer. Furthermore, it will be apparent that there is no requirement to synchronize the one PLL clock of multi-channel deserializer 230 to the incoming data channels, and that the incoming data channels to the deserializer may be transmitted with different clocks.

It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Claims (20)

1. A method for processing an input sequence of words received over a communication link, comprising:

assigning a respective bit-grade to at least one of the bits in a first word among the words in the input sequence;

deriving candidate words from the first word in response to the respective bit-grade;

inserting one of the candidate words into each of a plurality of candidate sequences, so that each of the candidate sequences contains one of the candidate words;

adding one or more subsequent words to each of the candidate sequences, each of the subsequent words consisting of a further candidate word derived from a further word received in the input sequence;

computing respective sequence parameters for the candidate sequences, based on a relation between the candidate words and the subsequent words in each of the candidate sequences;

selecting one of the candidate sequences in response to the sequence parameters; and

outputting the one of the candidate words contained in the selected one of the candidate sequences.

2. A method according to claim 1, and comprising assigning a respective word-grade to the one or more subsequent words in response to the one or more subsequent words and the respective sequence parameters, and wherein adding the one or more subsequent words comprises adding the one or more subsequent words to each of the candidate sequences in response to the respective word-grade.

3. A method according to claim 2, wherein the respective word-grade is operative as an index in a predetermined lookup table comprising entries for adding the one or more subsequent words.

4. A method according to claim 1, and comprising assigning a respective word-grade to the one or more subsequent words in response to the one or more subsequent words and the respective sequence parameters, wherein the plurality of candidate sequences comprises a first candidate sequence and a second candidate sequence, and comprising copying the first candidate sequence to the second candidate sequence in response to the respective word-grade.

5. A method according to claim 4, wherein the respective word-grade is operative as an index in a predetermined lookup table comprising entries for copying the first candidate sequence to the second candidate sequence.

6. A method according to claim 1, wherein selecting the one of the candidate sequences comprises assigning the one of the candidate sequences a highest probability of containing correct words.

7. A method according to claim 1, wherein the at least one of the bits comprises all the bits in the first word, and wherein assigning the respective bit-grade comprises determining respective higher probability values and lower probability values for all the bits, and wherein the candidate words comprise a first candidate word having all the higher probability values and a second candidate word having all the lower probability values.

8. A method according to claim 7, wherein the first and the second candidate words comprise respective first and second disparities, wherein computing the respective sequence parameters comprises computing respective running disparities for the candidate sequences, and wherein computing the respective sequence parameters comprises comparing the first and second disparities and the running disparities.

9. A method according to claim 7, wherein the first and the second candidate words comprise 10-bit words which are encoded from respective 8-bit words according to a predetermined standard.

10. Apparatus for processing an input sequence of words received over a communication link, comprising:

a processor which is adapted to:

assign a respective bit-grade to at least one of the bits in a first word among the words in the input sequence, and

derive candidate words from the first word in response to the respective bit-grade; and

a memory, which stores a plurality of candidate sequences, so that each of the candidate sequences contains one of the candidate words, the processor being further adapted to:

insert one of the candidate words into each of the plurality of candidate sequences,

add one or more subsequent words to each of the candidate sequences, each of the subsequent words consisting of a further candidate word derived from a further word received in the input sequence,

compute respective sequence parameters for the candidate sequences, based on a relation between the candidate words and the subsequent words in each of the candidate sequences,

select one of the candidate sequences responsively to the sequence parameters, and

output the one of the candidate words contained in the selected one of the candidate sequences.

11. Apparatus according to claim 10, wherein the processor is further adapted to assign a respective word-grade to the one or more subsequent words responsively to the one or more subsequent words and the respective sequence parameters, and to add the one or more subsequent words to each of the candidate sequences responsively to the respective word-grade.

12. Apparatus according to claim 11, wherein the respective word-grade is operative as an index in a predetermined lookup table comprising entries for adding the one or more subsequent words, and wherein the memory stores the lookup table.

13. Apparatus according to claim 12, wherein the processor is further adapted to assign a respective word-grade to the one or more subsequent words responsively to the one or more subsequent words and the respective sequence parameters, wherein the plurality of candidate sequences comprises a first candidate sequence and a second candidate sequence, and wherein the processor is further adapted to copy the first candidate sequence to the second candidate sequence responsively to the respective word-grade.

14. Apparatus according to claim 13, wherein the respective word-grade is operative as an index in a predetermined lookup table comprising entries for copying the first candidate sequence to the second candidate sequence, and wherein the memory stores the lookup table.

15. Apparatus according to claim 10, wherein the processor is adapted to assign the one of the candidate sequences a highest probability of containing correct words.

16. Apparatus according to claim 10, wherein the at least one of the bits comprises all the bits in the first word, and wherein the processor is adapted to determine respective higher probability values and lower probability values for all the bits, and wherein the candidate words comprise a first candidate word having all the higher probability values and a second candidate word having all the lower probability values.

17. Apparatus according to claim 16, wherein the first and the second candidate words comprise respective first and second disparities, and wherein the processor is adapted to compute respective running disparities for the candidate sequences and to compare the first and second disparities and the running disparities.

18. Apparatus according to claim 16, wherein the first and the second candidate words comprise 10-bit words which are encoded from respective 8-bit words according to a predetermined standard.

19. A method for processing an input sequence of words received over a communication link, comprising:

identifying a possible bit error in a first word among the words in the input sequence;

deriving candidate words from the first word responsively to the possible bit error;

inserting one of the candidate words into each of a plurality of candidate sequences, so that each of the candidate sequences contains one of the candidate words;

adding one or more subsequent words to each of the candidate sequences, each of the subsequent words consisting of either a further word received in the input sequence subsequent to the first word, or a further candidate word derived from the further word received in the input sequence;

computing respective sequence parameters for the candidate sequences, based on a relation between the candidate words and the subsequent words in each of the candidate sequences;

selecting one of the candidate sequences responsively to the sequence parameters; and

outputting the one of the candidate words contained in the selected one of the candidate sequences.

20. Apparatus for processing an input sequence of words received over a communication link, comprising:

a processor which is adapted to:

assign a respective quality-measure to at least one of the bits in a first word among the words in the input sequence, and

derive candidate words from the first word in response to the respective quality-measure; and

a memory, which stores a plurality of candidate sequences, so that each of the candidate sequences contains one of the candidate words, the processor being further adapted to:

insert one of the candidate words into each of the plurality of candidate sequences,

add one or more subsequent words to each of the candidate sequences, each of the subsequent words consisting of a further candidate word derived from a further word received in the input either a further word received in the input sequence subsequent to the first word, or a further candidate word derived from the further word received in the input sequence,

compute respective sequence parameters for the candidate sequences, based on a relation between the candidate words and the subsequent words in each of the candidate sequences,

select one of the candidate sequences responsively to the sequence parameters, and

output the one of the candidate words contained in the selected one of the candidate sequences.